Method for forming a dielectric film and novel precursors for implementing said method

ABSTRACT

The invention relates to dielectric layers with a low dielectric constant, said layers being used to separate metallic interconnections especially during the production of integrated circuit boards (in the BEOL part of the circuit). According to the invention, the dielectric layer comprises SiC and/or SiOC, and is obtained from at least one precursor comprising at least one —Si—C&lt;SUB&gt;n&lt;/SUB&gt;—Si chain where n=I.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of pending application Ser.No. 11/917,931 filed Dec. 18, 2007, which is a national stage entryunder 21 USC §371 of PCT/FR2006/001495 filed Jun. 21, 2006, which claimspriority to French application 0551675 filed Jun. 21, 2005, the entirecontents of each being incorporated herein by reference.

BACKGROUND

The present invention relates to a process for forming a dielectric filmthat can be used in the fabrication of semiconductors and to novelprecursors for implementing this process.

During the fabrication of integrated circuits, for example integratedcircuits for commercial applications or for microprocessors, a verylarge number of successive steps of selectively depositing under vacuumvarious layers of products are carried out, these successive stepsthemselves being separated by cleaning steps, for cleaning the siliconwafer (nowadays having a diameter of 300 mm) on which these successivelayers are stacked.

In general, two types of successively deposited layer may bedistinguished:

-   -   the first type of deposition or FEOL deposition essentially        consists in depositing a multitude of successive layers in order        to create active components of the field-effect transistor type        (deposition of photosensitive layer, masking, exposure to UV        radiation, cleaning, doping of the single-crystal layer        depending on the type of transistor produced, then deposition of        the electrodes, then deposition of the electrical contacts on        the drain, source and gate zones of each transistor, etc.); and

the second type of deposition or BEOL deposition consists in creating anetwork of electrical interconnections between the various electricalcontacts, especially semiconductors produced during the various steps ofthe first type of deposition, but also in creating, in particular in thecase of the fabrication of random-access memories, the capacitors neededfor recording information in digital form. In both these types ofapplication, the aim is essentially to deposit metallic and/orelectrically conducting layers, to deposit dielectric layers having avery low dielectric constant separating these conducting layers, and todeposit barrier layers, in particular for preventing diffusion into thelower layers or laterally when one or more lower layers, whetherdielectric or conducting, are etched using liquid chemicals or gaseousproducts while it being desirable for the etching to be selective.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a sectional view of an integrated circuit with itsvarious layers in its FEOL and BEOL parts.

DETAILED DESCRIPTION

The present invention essentially relates to the production ofdielectric layers in the case of the second (BEOL) type of depositionenvisioned above.

To increase the integration density of the circuits and correspondinglytheir data processing power in a smaller and smaller volume, it isnecessary to reduce the dimensions (thickness and length) of theinterconnect lines, thereby enabling them to be brought closer togetherand thus saving in volume, provided however that the dielectric layersthat separate, on the one hand, the electrical connections of twosuccessive layers in a vertical plane of the circuit, which layers arealso called ILD (interlayer dielectric) layers, and, on the other hand,the metal interconnects lying within any one horizontal plane of thecircuit, which layers are also called IMD (intermetal dielectric)layers, have improved dielectric properties (even lower dielectricconstant) in order to provide sufficient electrical insulation betweentwo connection lines that are closer together, but above all to reducethe time constant of the interconnect circuits (the capacitive componentof the time constant has a lower value the lower the dielectric constantof the dielectric, but on the contrary a higher value the closertogether its electrodes).

The ever increasing constraints imposed on the electrical interconnectwiring in the upper or BEOL part of the integrated circuit (see above)has recently led to important technological progress being made.

Copper has replaced aluminum as interconnect metal owing to its betterelectrical conductivity. This has led to the adoption of a new metaldeposition process using what is called a “dual-damascene” techniqueduring which a trench is created by the selective etching of aphotosensitive resin or photoresist, which trench is then filled withcopper using an electrodeposition process.

Moreover, films of SiOC (i.e. carbon-doped silicon oxide) are now widelyused as a replacement for the silicon dioxide used previously asdielectric in the interlayer dielectric and intermetal dielectric layersof an integrated circuit.

Finally, with the aim of lowering the dielectric constant of theselayers further, dielectrics having a porous structure have beenintroduced. SiO₂ dielectric films deposited by chemical vapor deposition(CVD), from a compound of silicon and oxygen, have a dielectric constantof about 4.0. The aim is to lower the dielectric constant of said layersso as to retain good insulating properties, while reducing theirthickness. Films having a low dielectric constant, or low-k films, thatis to say those made of materials having a dielectric constant of lessthan 4, may be produced in many ways, in particular by doping thesilicon oxide with organic ligands so as to obtain films having adielectric constant of between about 2.7 and 3.5. This approach, inwhich terminal organic ligands are used, that is to say those notconnected to the network of the molecule of the final material, leads tothe number of oxygen bonds (or “bridges”) between two silicon atoms ofthe —Si—O—Si— type being reduced, which therefore reduces the cohesionof the chain structure and consequently leads to a reduction in themechanical properties of this type of material compared with those ofSiO₂. However, these mechanical properties are critical for preventingthe successive layers of the integrated circuit from delaminating duringthe polishing process carried out in a subsequent step in thefabrication of the circuit (chemical mechanical polishing or CMP).Another important parameter to be considered in producing the dielectriclayers is the possible existence of internal tensile stresses in thestructure of the material constituting the film: most of the electronicstructure is created at a temperature of around 300 to 450° C. When theassembly is cooled, the materials, including the silicon substrate, thecopper network and the dielectric films, contract differently owing totheir different expansion coefficients. This creates a major source ofmechanical stresses that degrade the most brittle layer, namely thelow-k SiOC layer.

To deposit these low-k SiOC layers, there are presently two processes:

the chemical vapor deposition process, which uses precursors of thedimethyldimethoxysilane (DMDMOS), tetramethylsilane (4MS),tetramethylcyclotetrasiloxane (TMCTS) and octamethylcyclotetrasiloxane(OMCTS) type:

-   -   the spin-on deposition process, which uses siloxanes to form        films, such as methylsilsesquioxane (MSQ) or hydrogen        silsesquioxane (HSQ) in a suitable solvent.

During deposition of the metal interconnect layers and of the SiOClayers that separate and isolate them, it is sometimes necessary to alsodeposit a thin layer of SiC (called an etch-stop layer) for stopping thepassage of chemicals during a step of etching a layer depositedsubsequently on top of said etch-stop layer. Silicon carbide is arefractory insulating material having a dielectric constant of about 5,which obviously increases the average dielectric constant of thedielectric layer thus separating two metal interconnect layers.

At the present time it is endeavored to reduce the dielectric constantof these etch-stop layers, especially SiC layers.

The object of the present invention is to improve particularly themechanical properties of the dielectric layers, especially low-k layers,used in particular as interlayer dielectric layers or as intermetaldielectric layers for said BEOL electrical interconnection of thecomponents of an integrated circuit. To solve the technical problem thusstated, the present invention consists in using precursors of layers ofdielectric material containing at least one carbon atom and at least twosilicon atoms, the carbon and silicon atoms forming a chain of the—Si—C_(n)—Si— type with at least one carbon atom linked to two siliconatoms, where n is greater than or equal to 1, preferably less than orequal to 5.

According to a first embodiment, it will be possible to use precursorsof the alkoxysilylalkane (R^(i)O)₃(Si(—CR^(i) ₂)_(n)—Si(OR^(i))₃ type,in which formula each radical R^(i) may be chosen from hydrogen or acarbon chain, for example of the alkyl, aryl, etc. type, with a numberof carbon atoms between 1 and 5, preferably 1 or 2 carbon atoms, such asfor example BTESE (bis(triethoxysilyl)ethane) of formula(EtO)₃Si—CH₂—CH₂—Si(OEt)₃, where Et=C₂H₅, the dielectric constant andthe mechanical properties of which are substantially improved over thelayers produced with the precursors of the prior art.

According to another embodiment, the invention consists in using cyclicmolecules, which in particular make it possible to produce porousdielectric layers containing rings and chosen especially from one of thefollowing families:

R, R¹, R² being chosen from hydrogen, linear and/or cyclic carbonchains, such as alkyl chains, aryl chains, etc., or alkoxides.

According to the invention it will be preferable to choose moleculessuch as “double-DMDMOS” (formula I in which R═H and R¹=—O—CH₃). Morepreferably still, R will be chosen from the following groups: H, —CH₃ orin general an alkyl group, R¹ being chosen from —O—CH₃ or, moregenerally, an alkoxide or an amine. Other ligands for R¹, such as Cl, Meor H, may also be chosen.

All these molecules contain at least one carbon atom between two siliconatoms. Preferably, these molecules form a ring or a cage (athree-dimensional structure, illustrated by the molecular structure IV)containing at least one Si—C—Si chain (with the possibility of severalcarbon atoms between two Si atoms). This type of three-dimensionalmolecule favors the formation of pores in the films obtained, thusensuring both much greater structural cohesion than molecules withterminal alkyl groups and a very significant reduction in the dielectricconstant.

These various precursors may for example be synthesized by reactingdichlorosilylene: —SiCl₂ with hydrocarbons having π bonds, such asacetylene or butadiene. The dichlorosilylene source may behexachlorodisilane or trichlorosilane. It may be formed either at hightemperature by decomposition of hexachlorodisilane (HOD) or at lowtemperature from a solution containing for example a tertiary amine ascatalyst (such as trimethylamine). The synthesis of the molecules offormula I will for example be carried out in two steps:

the first step is common to all the molecules according to the reaction:

Si₂Cl₆ (or a mixture ofpolychlorosilanes)+ethylene→Cl₂Si—(CH═CH)₂—SiCl₂,

acetylene, butadiene, benzene, etc. leading to this type of reactionaccording to the same principle: dichlorosilylene: —SiCl₂ reacts withthe π bond. In general, all unsaturated species are potential candidatesfor synthesizing this type of molecule;

the second step is specific to the type of molecule in question:

Cl₂Si—(CH═CH)₂—SiCl₂+MeOH→(MeO)₂Si—(CH═CH)₂—Si(OMe)₂ (for molecule Iwith R¹═OMe);Cl₂Si—(CH═CH)₂SiCl₂+LiAlH₄ or NaBH₄→H₂Si—(CH═CH)₂—SiH₂ (for molecule Iwith R¹═H); andCl₂Si—(CH═CH)₂SiCl₂+Speyer catalyst→Me₂Si—(CH═CH)₂—SiMe₂ (for molecule Iwith R¹=Me).

It is also possible to obtain the fluorinated equivalent (for exampleF₂Si—(CH—CH)₂—SiF₂) with this method, it being possible for thisfluorinated equivalent to be used to form low-k films.

In general, a person skilled in the art will find all the informationneeded to synthesize these various products in, for example, thepublication by Atwell and Weyenberg, J. Am. Chem. Soc., 3438, 90, 1968.

According to the invention, the processes and the compositions meet theneed, for a person skilled in the art, of forming a thin insulatinglayer having excellent electrical and mechanical properties and alsohigh uniformity. The films deposited according to the invention areproduced by vaporizing a source of low-k precursors consisting of one ormore precursors for generating a vapor source of said precursors and bydelivering the vapors of these precursors into a deposition chamber inwhich the precursors are thermally decomposed and/or decomposed by theuse of a plasma, forming a film of the desired composition. The film isformed on one or more substrates in a single formation step without asubsequent heat treatment necessarily being required. The low-k filmthat results therefrom will have the desired composition, so as to havea low leakage current. In general, the precursors used in the processfor depositing films to the desired stoichiometry according to theinvention will be in liquid phase, for example a liquid precursor or aliquid solution of a precursor in a solvent, such as a hydrocarbon. Theprecursor in liquid phase is injected into a vaporization system, therate of vaporization of which is measured and controlled beforehand.This vaporized precursor is taken into the deposition chamber where thedeposition is carried out at a pressure of the order of 1 torr(generally less than 200 pascals) and at a temperature generally between0° C. and 450° C., preferably between 200° C. and 400° C. Optionally,the chamber is provided with a plasma for significantly increasing thedeposition rate. In general, but not necessarily, a coreactant is alsointroduced into the deposition chamber so as to react with theprecursor, this coreactant preferably being an oxidizing agent (oxygen,ozone, water vapor, hydrogen peroxide, alcohols, etc.).

According to the invention, the film is deposited by a CVD (chemicalvapor deposition) technique, such as the PECVD technique or the thermalCVD technique, these being well known to those skilled in the art,starting from at least one precursor, by itself or in combination with areactant, preferably an oxygen source. In general, a person skilled inthe art will optimize the precursor source by lowering the dielectricconstant (k) as much as possible while still maintaining an acceptablelevel of mechanical properties.

Preferably, the precursor is a molecule containing one or more carbonatoms between two silicon atoms. Even more preferably, this molecule iscyclic.

According to another aspect of the invention, the molecule is adisilane, which decomposes by thermal or plasma excitation and reactswith an unsaturated carbon chain (containing π bonds) to formintermediate species containing —Si—C bonds, preferably to formintermediate species containing a carbon or a carbon chain (whetherlinear or cyclic) linked to two silicon atoms, during the fabrication ofthe low-k film. Preferably, the carbons forming a carbon chainconnecting the silicon atoms will have at least one double bond becausethe C═C double bond is stronger than a C—C single bond. This allowslow-k films to be obtained that have mechanical properties improved overthe films obtained from a chain of carbon atoms containing only singlebonds.

Furthermore, the molecules described above that do not contain oxygenatoms have a molecular structure which is very suitable for depositing athin layer of SiC used as etch stop in the BEOL part of the integratedcircuit.

Preferably, the low-k SiC precursor is a molecule containing one or morecarbon atoms linked to two silicon atoms, but not containing oxygen.Even more preferably, the molecule is cyclic.

Preferably, the carbon atoms forming a carbon chain connecting thesilicon atoms have a double bond (in so far as a C═C double bond is moredifficult to break than a C—C single bond), thereby making it possibleto achieve high mechanical properties in the resulting films moreeasily.

According to another aspect of the present invention, the low-k SiCprecursor will be a disilane not containing oxygen, which decomposes bythermal or plasma excitation and reacts with an unsaturated carbon chain(containing π (pi) bonds) to form intermediate species containing Si—Cbonds, preferably to form intermediate species containing a carbon atomor a carbon chain linked to two silicon atoms, during the fabrication ofthe SiC film.

According to one other aspect, the invention relates to the dielectriclayers formed from the abovementioned precursors and to the use of theseprecursors for producing low-k dielectric layers.

The invention will be more clearly understood with the aid of thefollowing exemplary embodiments given by way of non-limiting exampletogether with the FIGURE, which shows a sectional view of an integratedcircuit with its various layers in its FEOL and BEOL parts.

In the FIGURE, the single-crystal silicon wafer 1 on which the variousMOS transistors with their interconnects and their protective layer areproduced represents the FEOL part of the integrated circuit, all of theupper layers beyond the stop layer 8 representing the BEOL part in whichthe electrical interconnects between the various circuits of the FEOLpart are produced as explained above.

Produced on the single-crystal substrate 1 is an integrated circuit,represented for example by 2, on which the drain contact (3), the gatecontact (4), the source contact (5) and the interconnects in thehorizontal level of this circuit, which interconnects are represented by(6), have been shown schematically. A vertical interconnect (7) made oftungsten or copper connects, in the sectional view shown in the FIGURE,a connection (6) through the stop layer (8) to the upper interconnectlevel (9) in which the copper connection (10), seen in cross section,extends in fact perpendicular to the plane of the FIGURE and is itselfconnected to the connection (12) at the upper level, then to theconnection (15) and then to the connections (15, 18, 28, 31, 34, 37)before terminating at the central connection (38) in the upperinterconnect part of the integrated circuit.

Between the various interconnect levels 9, 13, 16, 19, 29, etc. are therespective stop layers 11, 14, 17, 20, 30, 33, 36, 41, etc. whichseparate copper metal connections in various horizontal planes from oneanother, by means of ILD dielectric layers such as 21, 22, 23, 24, 25,26, 27. In a set of layers of any one level, such as for example the setlying between the two stop layers 30 and 33, there is, in the lowerpart, an ILD dielectric layer for providing isolation between twosuccessive horizontal copper interconnect layers and between the copperconnection sidewalls such as 44, 45, 46 and to have an IMD dielectriclayer so as to electrically isolate the copper electrical connectionssuch as 44 and 45.

In the example shown in the FIGURE, the ILD and IMD layers are producedfor example using the same SiOC dielectric.

The stop layers are in general SiC or SiN layers.

The invention relates to the formation of these interlayer dielectriclayers or ILDs and to the formation of intermetal dielectric layers orIMDs.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A low-k dielectric layer that can be used to separate metalinterconnects, in particular during the fabrication of integratedcircuits, wherein said layer comprises SiC and/or SiOC and is obtainedfrom at least one precursor containing at least one —Si—C_(n)—Si-chain,where n≧1, the at least one precursor having a formula I:

wherein the precursor has formula I selected from the group consistingof: i) formula I in which R═H and R¹=—O—CH₃, ii) formula I in which R is—CH₃ and R¹=—O—CH₃, iii) formula I in which the precursor is

and combinations thereof.
 2. A precursor molecule, especially an SiC orSiOC precursor molecule, of general formula: